It is desirable to be able to measure the gaseous ammonia concentration in a variety of environments. In particular, it is often desirable to continuously monitor gaseous ammonia in situ in such environments as flue gas, chemical plant feedstreams and atmospheric backgrounds. In fact, it has been mandated to continuously monitor the ammonia concentration in flue gases resulting from deNox processes which introduce ammonia to the combustion products in order to accomplish conversion of oxides of nitrogen to N.sub.2 and H.sub.2 O. In such processes the level of ammonia which must be detected is as low as 5 parts per million.
Using standard techniques which require point sampling of the gases poses the difficult problem of avoiding plugging of sampling systems, which may happen by physical blockages, due to e.g., fly ash in coal-fired boilers, or by deposition of chemical reaction products (e.g. ammonium-sulfates or sulfites) in the sampling system. This latter problem may be alleviated by raising the temperature of the sampling system, but the possibility of residual reactions causing changes in ammonia concentration during transport of the gaseous sample from a flue duct to a measuring system introduces an uncertainty into how accurately the same as analyzed represents the flue gas. In addition, techniques which sample a single point assume a homogeneous ammonia distribution within the sampled ducts.
In the present invention, infrared laser absorption is employed as a remote diagnostic of ammonia concentration, in part to obviate problems associated with sampling systems.
It is known that optical absorption due to ammonia depends upon optical wavelength, ammonia concentration, and optical path length, and that absorption of optical radiation from lasers can be used to detect the presence of ammonia in an atmospheric background.
In an article entitled "Measurements of NH.sub.3 Absorption Coefficients With a C.sup.13 O.sub.2.sup.16 Laser", by Messrs. Allario and Seals, appearing in Applied Optics, Volume, 14, No. 9, September 1975 at page 2229 (which article is incorporated herein by reference as though fully set forth herein) it was found that absorption of certain C.sup.13 O.sub.2.sup.16 laser wavelengths could be directly related to ammonia concentration. It was also found that certain other laser transitions were only negligibly absorbed by ammonia in an atmospheric background, and it was noted that transmission of these wavelengths could be used to distinguish ammonia absorption from scattering or absorption by atmospheric particulates or variations in refractive index, since such interfering mechanisms would attenuate all CO.sub.2 laser wavelengths nearly equally.
The prior art does not provide a prescription for the construction of a device useful for monitoring trace NH.sub.3 concentrations in a hot flue gas background, for several reasons. First, the coefficient relating optical absorption by NH.sub.3 molecules to their concentration depends on both the line strength and the spectral width of the probed NH.sub.3 line. The line strength depends on a population difference and thus on temperature. The line width varies with both temperature and composition (percentages of H.sub.2 O and CO.sub.2) of the flue gas, which in turn change with the type of combustion, amount of excess air, and location of sampling point. Furthermore, the lowest level of NH.sub.3 which can be detected is limited by the existence of a small difference in absorption from the H.sub.2 O and CO.sub.2 present in the flue gas itself at the two wavelengths selected for monitoring. This residual absorption represents a zero level for the NH.sub.3 measurement. As the flue gas composition and temperature vary, so does this zero level, thereby limiting the sensitivity of the NH.sub.3 detection.
One would therefore assume that both the composition and the temperature of the flue gas must be monitored together with optical absorption to determine NH.sub.3 concentration with an adequate degree of sensitivity and accuracy. A system capable of monitoring CO.sub.2 and H.sub.2 O concentration, temperature, and optical absorption would be so complex and costly as to render the method impractical. The present invention demonstrates, however, that measurements of flue gas temperature and optical absorption suffice to determine NH.sub.3 concentrations in flue gas to the 5 ppm level even if the flue gas H.sub.2 O and CO.sub.2 composition is uncertain within typical limits.